1. Cross-Reference
This application is related to U.S. Pat. No. 4,931,528.
2.Field of the Invention
This invention relates to epoxy resin curing agents. More particularly this invention relates to the composition 1-isopropyl-2-tolylimidazole and to its use in curing epoxy resin systems and providing stable one-component mixtures with a high degree of resistance to heat and chemicals. Further, epoxy systems cured with this novel curative demonstrate a low degree of reactivity and lower viscosities at ambient temperatures and increasingly accelerated rates of reactivity at moderately elevated temperatures, thus providing a variety of application possibilities.
3. Related Art
Epoxy resins include a broad class of polymeric materials having a wide range of physical properties. The resins are characterized by epoxide groups which are cured by reaction with certain catalysts or curing agents to provide cured epoxy resin compositions with various desirable properties.
The most common epoxy resins are condensation products of epichlorohydrin and the diglycidyl ether of Bisphenol A (DGEBA). These systems can be cured with conventional curing agents such as polyamines, polycarboxylic acids, anhydrides and Lewis acids. Bisphenol A based epoxy compositions, when cured, have good adhesive properties, however many are inherently stiff and brittle and hence their use is limited to applications where peel forces do not come into play.
It is known in the art to use some substituted imidazoles as curing agents for epoxy resins. In [95] Chemical Week, July 31, 1965 it is stated imidazoles generally offer, when used at low concentration, HDT's to 150.degree. C. with DGEBA and other elevated-temperature properties in the range expected of aromatic amine cures. This improved heat resistance is obtained with a super-cooled liquid having a viscosity of 4000 to 8000 centipoises and providing a long pot life. Generally, imidazoles are in solid form and cause predictable results with the resin to be cured. For instance, one could expect a substantial amount of gelling of the resin at lower temperatures.
It is known to use salts of imidazoles and alkyl imidazoles as semi-latent epoxy curatives as described in data sheets from Shikodu titled ##STR1## In that work what is disclosed is curatives consisting of isocyanuric acid addition compounds in the form of crystals. Such salts are usually solid products which either melt or decompose at rather high temperatures.
2-Ethyl-4-methylimidazole is described in the chemical literature as an epoxy curative. See Handbook of Epoxy Resins, Lee and Neville, Ref. 95, Ch 10 (1965).
Japanese Patent 633 080 30-A describes hardeners containing 1-alkyl-2-phenylimidazoles where the alkyl group contains 1-6 carbons.
New curing agents for epoxy resins are useful and desirable, especially if they demonstrate unusual and useful properties. It would be an advance in the art to provide an imidazole epoxy resin curing agent which would easily dissolve and mix with many epoxy systems. Further, it would be an advance in the art if the curative added latency to the epoxy systems to which it was added. If such a curative provided a formulation which was less viscous than formulations using related curing agents, such that a much longer time was available for utilization of the curing agent after mixing, this would constitute an improvement over what is available in the art. If such a system, when fully cured could demonstrate acceptable physical properties with exceptional resistance to heat and chemicals, it would meet a number of needs in the art. If a curative with the above properties also allowed for variations in reactivity over a broad range of temperatures and differences in physical state depending upon whether the ortho, meta or para form were used, this would be particularly desirable and a variety of potential applications would be apparent to those skilled in the art.
It has now been discovered that the composition comprising 1-isopropyl-2-(ortho, meta or para)-tolylimidazole can be employed in liquid form as an epoxy resin curative to provide these and other desirable properties.